Re: [PATCH] mm/damon: update expired description of damos_action

[Honggyu Kim <honggyu.kim@sk.com>]

Hi SeongJae and Sang-Heon,

On 8/2/2025 1:50 AM, SeongJae Park wrote:
> On Sat, 2 Aug 2025 01:11:09 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> 
>> Hi, Honggyu
>>
>> On Fri, Aug 1, 2025 at 8:35 PM Honggyu Kim <honggyu.kim@sk.com> wrote:
>>>
>>> Hi Sang-Heon and SeongJae,
>>>
>>> On 8/1/2025 2:58 AM, SeongJae Park wrote:
>>>> Hello Sang-Heon,
>>>>
>>>> On Thu, 31 Jul 2025 22:22:30 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
>>>>
>>>>> Nowadays, damos operation actions support more various operation set.
>>>>> But comments(also, generated documentation) doesn't updated.
>>>>> So, fix the comments with current support status.
> [...]
>>>>> diff --git a/include/linux/damon.h b/include/linux/damon.h
> [...]
>>>>>     * @DAMOS_WILLNEED:  Call ``madvise()`` for the region with MADV_WILLNEED.
>>>>>     * @DAMOS_COLD:              Call ``madvise()`` for the region with MADV_COLD.
>>>>> - * @DAMOS_PAGEOUT:   Call ``madvise()`` for the region with MADV_PAGEOUT.
>>>>> + * @DAMOS_PAGEOUT:   Reclaim the region.
>>>>
>>>> Nice!
>>>
>>> But doesn't it make confusion about whether this pages out to disk or does
>>> demotion to the lower tier memory?  It's because PAGEOUT action doesn't do
>>> demotion, but it looks "reclaim" includes pageout and demotion together in my
>>> understanding since /sys/kernel/mm/numa/demotion_enabled was introduced.
> 
> To my understanding, DAMOS_PAGEOUT can also do demotion when demotion_enabled
> is set.  Am I missing something?

Actually no, please see below.

do_demote_pass in shrink_folio_list()
https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L1122

The do_demote_pass is used here.
https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L1293-L1302

can_demote() implementation returns false when demotion_enabled is on.
https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L350-L351

The replated commit is as follows.
mm/migrate: add sysfs interface to enable reclaim migration
https://github.com/torvalds/linux/commit/20b51af15e014cac63b58a4f8b8b323ac35bccce

> 
>>
>> My intention was just to synchronize with the Design documentation.
>>
>> So how about changing the description to `Page out the region`, Would
>> this be also confusing?
>> I feel like it would be clearer than using word "reclaim"

I don't have a good idea but it looks like recusive explanation.

> 
> In my opinion, "reclaim" is good.

I wish there could be better term that distinguishes between swap out and
demotion.  In my understanding "reclaim" includes both swap out and demotion.

I also found that man page explanation about MADV_PAGEOUT is "reclaim these 
pages".  If this is correct, then maybe demotion isn't included in "reclaim".

Thanks,
Honggyu